diff --git a/packages/rstack/src/fmt/cli.ts b/packages/rstack/src/fmt/cli.ts
index f4c9640..ec47edf 100644
--- a/packages/rstack/src/fmt/cli.ts
+++ b/packages/rstack/src/fmt/cli.ts
@@ -253,7 +253,7 @@ const loadFmtConfig = async (
     loadedConfig ?? (await loadRstackConfig({ cwd }));
 
   return resolveFmtConfig({
-    definition: configs.fmt,
+    layers: [configs],
     configFilePath: filePath,
     cwd,
   });
diff --git a/packages/rstack/src/fmt/config.ts b/packages/rstack/src/fmt/config.ts
index 5e4c543..54d735b 100644
--- a/packages/rstack/src/fmt/config.ts
+++ b/packages/rstack/src/fmt/config.ts
@@ -1,15 +1,16 @@
 import { dirname } from 'node:path';
 import micromatch from 'micromatch';
+import type { Configs } from '../config.ts';
+import { resolveConfigLayers } from '../configLayers.ts';
 import { createRelativePathResolver } from './pathHelpers.ts';
 import type {
   FmtConfig,
-  FmtConfigDefinition,
   ResolvedFmtConfig,
   ResolvedFmtOptions,
 } from './types.ts';
 
 type ResolveFmtConfigOptions = {
-  definition: FmtConfigDefinition | undefined;
+  layers: readonly Configs[];
   configFilePath: string | null;
   cwd: string;
 };
@@ -150,14 +151,17 @@ const createOptionsResolver = (
   };
 };
 
-/** Resolves a formatter config definition and its project root. */
+/** Resolves formatter config layers and their project root. */
 const resolveFmtConfig = async ({
-  definition,
+  layers,
   configFilePath,
   cwd,
 }: ResolveFmtConfigOptions): Promise<ResolvedFmtConfig> => {
+  const configs = await resolveConfigLayers(layers, 'fmt');
   const config =
-    typeof definition === 'function' ? await definition() : definition;
+    configs.length > 1
+      ? (Object.assign({}, ...configs) as FmtConfig)
+      : configs[0];
   const rootPath = configFilePath ? dirname(configFilePath) : cwd;
 
   return normalizeFmtConfig(config, rootPath);
diff --git a/packages/rstack/tests/fmt/config.test.ts b/packages/rstack/tests/fmt/config.test.ts
index eb5a9b2..978f135 100644
--- a/packages/rstack/tests/fmt/config.test.ts
+++ b/packages/rstack/tests/fmt/config.test.ts
@@ -3,10 +3,57 @@ import { expect, test } from 'rstack/test';
 import {
   createOptionsResolver,
   normalizeFmtConfig,
+  resolveFmtConfig,
 } from '../../src/fmt/config.ts';
 
 const rootPath = path.join(import.meta.dirname, 'project');
 
+test('shallowly merges fmt layers before normalizing options and overrides', async () => {
+  const config = await resolveFmtConfig({
+    layers: [
+      {
+        fmt: {
+          singleQuote: true,
+          semi: true,
+          plugins: ['shared-plugin'],
+          ignorePatterns: ['dist/**'],
+          overrides: [{ files: '*.ts', options: { tabWidth: 8 } }],
+          pluginOptions: { shared: true, project: false },
+        },
+      },
+      {
+        fmt: () =>
+          Promise.resolve({
+            semi: false,
+            plugins: ['project-plugin'],
+            ignorePatterns: ['generated/**'],
+            overrides: [{ files: '*.ts', options: { semi: true } }],
+            pluginOptions: { project: true },
+          }),
+      },
+    ],
+    configFilePath: path.join(rootPath, 'rstack.config.ts'),
+    cwd: path.dirname(rootPath),
+  });
+
+  expect(config).toEqual({
+    rootPath,
+    baseOptions: {
+      singleQuote: true,
+      semi: false,
+      plugins: ['project-plugin'],
+      pluginOptions: { project: true },
+    },
+    ignorePatterns: ['generated/**'],
+    overrides: [
+      {
+        matches: expect.any(Function) as unknown,
+        options: { semi: true },
+      },
+    ],
+  });
+});
+
 test('reuses base options when no override matches', () => {
   const config = normalizeFmtConfig(
     {